High-pressure phase boron nitride represented by cubic boron nitride (hereinafter referred to as cBN) is the next high-hardness material to diamond, and its reactivity with a ferrous material is low as compared with diamond, and hence cBN-based sintered bodies are used for various cutting tools.
Further, a material which is of high hardness and high strength is earnestly desired not only in a cutting application but also in various applications such as a wear-resistant part provided on a sliding part and a shock-resistant part employed for a defensive wall, for example. In a conventional high-pressure phase boron nitride-based sintered body, however, there has been a problem in compatibility of hardness and strength, and no sufficient performance has been attained.
At this point, cBN is a typical non-sinterable material ranking with diamond, and is a high-pressure stable phase. Therefore, extreme sintering conditions of 2000.degree. C. and at least 8 GPa are required, in order to sinter cBN grains. Therefore, cBN grains cannot be bonded to each other under industrial sintering conditions of 1450.degree. C. and not more than 4.5 GPa. Therefore, it is necessary to sinter cBN powder and binder powder after mixing, in order to prepare a cBN-based sintered body under industrial sintering conditions. Powder of Al, Ti Al, Ti Al.sub.3, TiN or TiC is employed as this binder powder. And, cBN-based sintered bodies industrially produced by employing a binder (hereinafter referred to as an Al-based binder) consisting of an Al metal or an intermetallic compound of at least one of Al elements can be roughly classified into the following two types:
It is disclosed in Japanese Patent Laying-Open Gazette No. 55-126581 that a cBN-based sintered body (A) consisting of at least 80 weight % of cBN and a binder phase is obtained by performing sintering while employing cBN grains and Al as starting materials. This is because metallic Al or an intermetallic compound of Al such as Ti Al.sub.3 causes fused Al in a high-temperature state in sintering and promotes formation of neck growth between the cBN grains. At this point, neck growth indicates such a state that the cBN grains are fused or bonded and a continuous mosaic or an alternate material is generated. A cBN-based sintered body having a cBN content of 85 to 90 volume % is worked into a product in practice. The transverse rupture strength of this cBN-based sintered body is about 80 to 100 kgf/mm.sup.2 under a condition of a 4 mm span with a test piece of 6 mm in length, 3 mm in width and 0.4 to 0.45 mm in thickness.
On the other hand, a cBN-based sintered body (B) consisting of about 50 to 80 volume % of cBN and a binder phase is obtained by performing sintering while employing cBN grains, an Al-based binder and a nitride or a carbide of an element of the group 4a, 5a or 6a of the periodic table represented by TiN and TiC and the like or a solid solution thereof (hereinafter referred to as a transition metal nitride or the like) as starting materials. This is because metallic Al or an intermetallic compound of Al such as TiAl.sub.3 causes fused Al in a high-temperature state in sintering, forms reaction products between the cBN grains and grains of the transition metal nitride or the like and between the grains of the transition metal nitride or the like, and forms strong binding. A cBN-based sintered body having a cBN content of about 50 to 80 volume % is worked into a product in practice, as a high-strength cutting tool employed for an intermittent cutting application or the like. The transverse rupture strength of this cBN-based sintered body is about 90 to 110 kgf/mm.sup.2 under a condition of a 4 mm span with a test piece for measurement of 6 mm in length, 3 mm in width and 0.4 to 0.45 mm in thickness.
The theoretical strength of the cBN grains is about 70 GPa when estimated from the Young's modulus. Further, the theoretical strength of the grains of the transition metal nitride or the like is about 20 to 50 GPa. In reality, however, the aforementioned cBN-based sintered body (A) is lower in transverse rupture strength than the aforementioned cBN-based sintered body (B), although the content of the cBN grains having high theoretical strength is high. Namely, the aforementioned cBN-based sintered body (B) is of higher strength than the aforementioned cBN-based sintered body (A) having neck growth between the cBN grains as the main of the bonding form of the constituent grains. Thus, it is understood that the bond strength between the cBN grains and the grains of the transition metal nitride or the like and between the grains of the transition metal nitride or the like is stronger than the bond strength by the neck growth between the cBN grains.
However, the aforementioned cBN-based sintered body (B) is prepared by mixing and charging the Al-based binder, the cBN powder and the transition metal nitride or the like and thereafter sintering the same, as hereinabove described. The Al-based binder has a function of neck-growing the cBN grains, as hereinabove described. In the conventional mixing state, therefore, a region where the cBN grains are in contact with each other through the Al-based binder and a region where the cBN grains are directly in contact with each other and the Al-based binder exists in the vicinity thereof have been present to no small extent. Therefore, a region where the aforementioned cBN grains cause neck growth has been generated in sintering. Consequently, holding power for the cBN grains weakens due to the occurrence of the neck growth also in the aforementioned cBN-based sintered body (B), and there has been such a problem that sufficient wear resistance and chipping resistance have not been exhibited when compared with an ideal cBN-based sintered body.
There are Japanese Patent Laying-Open Gazette No. 58-58247, Japanese Patent Laying-Open Gazette No. 58-60678, Japanese Patent Laying-Open Gazette No. 5-186844 and Japanese Patent Laying-Open Gazette No. 58-61253 as those proposed in order to solve such a problem.
In Japanese Patent Laying-Open Gazette No. 58-58247, there is disclosed a high-toughness boron nitride-based sintered body for cutting and wear-resistant tools comprising cBN or wurtzite boron nitride (hereinafter referred to as wBN) and a binder phase. The aforementioned binder phase consists of a boride and a carbide of at least one of Ti, Hf, Zr and Mo. At least either the aforementioned cBN or wBN is enclosed with the aforementioned boride which is 0.1 to 2 .mu.m in mean thickness.
In Japanese Patent Laying-Open Gazette No. 58-60678, there is disclosed a high-toughness boron nitride-based sintered body for cutting and wear-resistant tools comprising at least either cBN or wBN and a binder phase consisting of a nitride and a carbide of at least one of Ti, Hf and Si. At least either the aforementioned cBN or wBN is enclosed with the aforementioned boride whose mean thickness is 0.1 to 2 .mu.m.
In Japanese Patent Laying-Open Gazette No. 5-186844, further, there is disclosed a sintered body containing high-density phase boron nitride, comprising at least either cBN or wBN and a binder phase consisting of a carbide, a nitride, an oxide or a boride of a metal of the group 4a, 5a or 6a of the periodic table, Al, Si, Fe, Ni or Co, an oxide or a nitride of a rare earth metal or a solid solution thereof, or Fe, Ni and Go. The aforementioned sintered body is obtained by sintering a composite hard phase prepared by coating at least either cBN or wBN with at least one of a nitride and a boride of Ti, Hf, Zr, Mo, Al or Si and a solid solution of these having a mean thickness of 0.5 to 90 nm.
In Japanese Patent Laying-Open Gazette No. 58-61253, further, there is disclosed a high-toughness material boron nitride-based sintered body for cutting and wear-resistant tools, which is of a composition containing at least either cBN or wBN and one or two of Al and an oxide and a nitride of Al. The aforementioned sintered body has such a structure that Al or one or two of Al and an oxide and a nitride of Al whose mean layer thickness is 0.1 to 1 .mu.m encloses at least either the aforementioned cBN or wBN.
In the high-pressure phase boron nitride-based sintered bodies described in the aforementioned Japanese Patent Laying-Open Gazette No. 58-58247, Japanese Patent Laying-Open Gazette No. 58-60678 and Japanese Patent Laying-Open Gazette No. 5-186844 high-pressure phase boron nitride grains of at least either cBN or wBN are coated with binders and sintered. Thus, the cBN grains in the sintered bodies aggregate thereby reducing regions being unsintered and improving wear resistance and chipping resistance.
Further, the sintered body described in Japanese Patent Laying-Open No. 58-61253 is such a one that Al enclosing at least either cBN or wBN counter-diffuses with and is strongly bonded to high-pressure phase boron nitride such as cBN and the oxide and the nitride of Al, thereby improving toughness of the aforementioned sintered body.
Grain growth of the binder phase has been a serious problem in the aforementioned generally proposed high-pressure phase boron nitride-based sintered bodies employing high-pressure phase boron nitride coated with the binders, not to mention the conventional high-pressure phase boron nitride-based sintered body. Namely, even if a high-pressure phase boron nitride-based sintered body having such a structure that high-pressure phase boron nitride grains homogeneously disperse in the aforementioned binder phase is prepared, there has been such a problem that the continuous binder phase causes conversion to coarse grains by grain-growing during sintering and chipping resistance lowers.
In the sintered bodies described in the aforementioned Japanese Patent Laying-Open Gazette No. 58-58247, Japanese Patent Laying-Open Gazette No. 58-60678 and Japanese Patent Laying-Open Gazette No. 5-186844, the binders containing elements such as Al, Ti and Hf coat the high-pressure phase boron nitride grains as at least one selected from nitrides, borides and solid solutions thereof. The aforementioned binders coat the high-pressure phase boron nitride grains as thermally and chemically stable ceramic, and hence bond strength formed between the high-pressure phase boron nitride grains and the grains of the transition metal nitride or the like and between the grains of the transition metal nitride or the like is weak as compared with the Al-based binder causing fused Al in a high-temperature state in sintering. Thus, these have been unsatisfactory in wear resistance and chipping resistance, following harshening of cutting conditions following recent requirement for efficiency increase-high-speeding.
In the sintered body described in the aforementioned Japanese Patent Laying-Open Gazette No. 58-61253, on the other hand, most part of Al remains as metallic Al in an unreacted state after sintering, and hence it has not been practical in such an application that the cutting temperature readily reaches a level exceeding 1000.degree. C. in case of cutting hardened steel or cast iron or the like, since metallic Al is fused.
Further, the wBN grains employed in the aforementioned proposed sintered body are synthesized by an impact compression method, while cBN grains are synthesized by a static compression method. Both of wBN and cBN are high-pressure phase boron nitride, and various physical properties such as hardness, chemical stability and reactivity with the Al-based binder in the grains themselves are substantially equivalent. However, they remarkably differ from each other in existence forms of the grains thereof respectively. Namely, while the cBN grains mainly consist of single crystals, wBN is polycrystalline grains consisting of secondary grains in which primary grains having grain sizes of several 10 nm to several 100 nm are sintered by energy in impact compression. The grain sizes of these secondary grains of wBN reach about several .mu.m.
Even if wBN is coated with the binder, therefore, it comes to that strong bonding through the grains of the transition metal nitride or the like is formed only between primary grains of wBN located on surfaces of the secondary grains of wBN and primary grains of wBN located on surfaces of other cBN grains or other secondary grains of wBN. Consequently, the primary grains of wBN are bonded to each other not through the binder in the interior of the secondary grains of wBN, and it could not be said that the same has sufficient wear resistance and chipping resistance.
The present invention has been proposed in order to solve the aforementioned problems, and aims at providing a high-pressure phase boron nitride-based high-hardness high-strength sintered body for cutting tools represented by a milling tool and an end mill and the like, which is improved in wear resistance and chipping resistance.